Ink jet printing systems using an intermediate imaging member are well known. Generally, the printing or imaging member is employed in combination with a print head to generate an image with a marking material, such as ink. The ink is typically applied to an imaging member, such as a drum or belt, by the nozzles of the print head to form an image on the imaging member as it rotates. After the ink is deposited onto the imaging member to form the image, a sheet of print medium is removed from a media supply and fed to a nip between the imaging member and a transfer roller. As the imaging member rotates, the print medium is pulled through the nip and pressed against the deposited image on the imaging member, thereby transferring the image to the print medium.
Efficient transfer of a marking material from an intermediate imaging member to a media sheet is enhanced by heating a media sheet before it is fed into the nip for transfer of the image. Preheating of the recording medium typically prepares the recording medium for receiving ink by driving out excess moisture that can be present in the recording medium. Preheating the medium reduces the amount of time necessary to dry the ink once deposited on the recording medium. Preheating may also reduce paper cockling which can result from excess moisture remaining in the recording medium.
Previously known preheaters typically a metallic support plate to which a pattern of heat traces have been laminated. The support plate is located in the path of the media to engage and heat the media immediately prior to its engagement with the intermediate transfer drum.
One practical challenge in the design of a preheater is maintenance of a consistent, or uniform, temperature at the heating surface of the preheater. Laminating techniques may leave air gaps between the layers and these gaps make uniform heating difficult. Additionally, insufficient bonding between the layers may cause delamination. Entrapped air and insufficient bonding may lead to stress cracks that can limit the heating element's ability to generate heat homogeneously, which tends to create hot and cold spots along the length of the element.
Non-uniform heating of the media may cause the production of the images by the printer to also be non-uniform. For example, uneven drying and shrinkage of the media may affect the quality of the images produced by the printer. Uneven shrinkage causes the paper to buckle in places which may vary the orientation of the media to the image on the intermediate imaging member in the nip. These unpredictable variations in distance and angle reduce print quality.
Previously known systems for monitoring the temperature of a preheater typically involved one or more temperature sensors, such as thermocouples or thermistors, mounted to the support plate and electrically connected to a conventional proportional temperature controller. Thermocouples and thermistors, however, are only capable of detecting the temperature of the preheater at relatively small areas of the plate. In order to detect inconsistencies in temperature along the entire surface area of the preheater, many thermocouples would be needed which greatly increases the hardware cost and complexity of the system.